1. Field of the Invention
The present invention relates to a compound for light emitting device and an organic light emitting device using the same.
2. Description of the Background Art
The organic electroluminescent device (hereinafter abbreviated as an organic EL device) is expected as a new type self light emitting device. The organic EL device has a layered structure including a carrier transport layer (electron or hole transport layer) and a light emitting layer between a hole injection electrode and an electron injection electrode. An electrode material having a large work function such as gold (Au) or ITO (Indium-Tin Oxide) is used for the hole injection electrode, while an electrode material having a small work function such as Mg (magnesium) or Li (lithium) is used for the electron injection electrode.
An organic material is used for each of the hole transport layer, light emitting layer, and electron transport layer. A material having the characteristic of p-type semiconductor is used for the hole transport layer, while a material having the characteristic of n-type semiconductor is used for the electron transport layer. The light emitting layer also has carrier transport capability such as electron or hole transport capability, and is made of an organic material emitting fluorescent light or phosphorescent light.
The hole injection electrode, hole transport layer, light emitting layer, electron transport layer, and electron injection electrode are layered in this order to form a device. Note that depending upon organic materials to be used, these functional layers such as the hole transport layer, electron transport layer and light emitting layer may each include a plurality of layers or some of them may not be provided at all.
As examples of such device structures, there may be mentioned: a structure in which only two organic layers, the light emitting layer and electron transport layer exist between the hole injection electrode and electron injection electrode; a structure in which only two organic layers, the hole transport layer and light emitting layer exist; and a device structure in which only three organic layers, the hole injection layer, hole transport layer, and light emitting layer exist. The device structure may be adjusted according to the characteristics of the material used for each organic layer.
The organic EL device can provide a visible light ranging from blue to red by selecting an organic material forming the light emitting layer. Therefore, through the use of organic EL devices emitting respective monochromatic lights of red, green, and blue which are three primary colors of light (RGB), a full-color display is realized.
By the way, among the red, green, and blue lights obtained by the organic EL devices, green and blue lights are stable.
M. A. Baldo et al., for example, has reported that the employment of iridium(III)tris(2-phenylpyridinato)-N,C2 (hereinafter abbreviated as Ir(ppy)3) led to the achievement of high efficient luminescent characteristics (see, M. A. Baldo et al., Applied Physics Letters, Vol. 75, No. 1, p4, (1999)). The molecular structure of Ir(ppy)3 is expressed in the following formula (4):

As expressed in the formula (4), Ir(ppy)3 is a complex of phenylpyridine and iridium metal, emitting green light.
On the other hand, it is difficult to obtain orange-red light having high luminance and luminous efficiency. This is because there is no solid organic material that effectively emits fluorescent or phosphorescent light of orange-red color.
With relation to the above, a method has been proposed in which the luminescent wavelength of an organic EL device is shifted to a longer wavelength in order to effectively obtain an orange-red emission.
There is, for example, a method in which a heterocyclic ring structure with a small energy gap is employed for a ligand of a complex forming an emitting material. As for this method, a case using an iridium complex having a ligand of a derivative, such as benzothiophene or benzothiazole as an emitting material, has been reported (see, S. Lamansky et al., J. Am. Chem. Soc., 123, 4304-4312 (2001)). Using this kind of emitting material, a maximum luminescent wavelength can be shifted to a longer wavelength.
Such an emission spectrum, however, has a broad shape than the emission spectrum of Ir(ppy)3 emitting green light. This may inhibit achievement of an emission color with good purity, causing the emission to extend out of a visible range (near infrared radiation range).
In addition, there is a method in which a ligand having a fused ring structure is employed for the complex forming a emitting material, so that the π conjugated system is extended to make an energy gap smaller. As for this method, a case using a complex having a ligand of a fused polycyclic compound, such as benzoquinoline or phenylquinoline as an emitting material, has been reported (refer to S. Lamansky et al., J. Am. Chem. Soc., 123, 4304-4312 (2001)). Using such emitting material, a maximum luminescent wavelength can be shifted to a longer wavelength.
However, in this case also, the emission spectrum results in a broader shape than the emission spectrum of Ir(ppy)3 emitting green light. As a result, light of high color purity is not obtained.